This invention relates to a method for controlling a crankcase scavenged engine during a deceleration fuel cut-off operating mode, and more particularly, to regulating the quantity of air inducted by such an engine, during deceleration fuel cut-off operation, to achieve increased engine braking.
It is customary to interrupt the supply of fuel to all cylinders of a four-stroke engine, during a mode of engine operation commonly known as deceleration fuel cut-off (DFCO). The DFCO mode is generally initiated when an engine powered vehicle is decelerating, and its engine output control element (throttle valve or accelerator pedal) is positioned for engine idling, i.e. no operator demand for additional engine output. Recovery from the DFCO mode typically occurs, when either the engine rotational speed drops below a predetermined minimum speed near idle, or the engine control element is moved from the idling position to accelerate engine rotation and increase output torque.
The purpose of the DFCO operating mode is to reduce fuel consumption, and maximize the engine braking that results from frictional drag and negative torque applied to the engine by its load. In addition, the customary closing of the air throttle valve during DFCO, reduces exhaust emissions from engines equipped with catalytic converters. Restricting engine air flow, during DFCO, decreases the cooling rate of the exhaust catalyst, thereby extending the period of time that the catalyst remains at efficient operating temperatures.
When a conventional crankcase scavenged two-stroke engine is operated in the DFCO mode, according to customary practice, the realized engine braking is substantially reduced from that achieved with four-stroke engines. This is particularly noticeable, when a vehicle equipped with a crankcase scavenged two-stroke engine is operated on extended negative grades. Here, the reduced engine braking is perceived as inadequate deceleration, which significantly degrades the drivability of vehicles employing crankcase scavenged, two-stroke engines. Consequently, a need exists for a method of increasing engine braking, when a crankcase scavenged, two-stroke engine is operated in the deceleration fuel cut-off mode.